Nanocomposites have recently attracted considerable attention as a means of enhancing polymer properties. In general there are two types of nanocomposite structures, intercalated and exfoliated. Such structures are classified according to the separation of the clay layers. In an intercalated nanocomposite, extended polymer chains are inserted between the host clay layers resulting in a well ordered multilayer nanostructure where the insertion of a polymer into the clay structure occurs in a crystallographically regular fashion regardless of the clay to polymer ratio. Intercalation of various monomers into the silicate galleries, followed by in-situ polymerization, yields highly oriented multi-layers consisting of the polymer alternately stacked with the layers of the host.
The host layers of exfoliated nanocomposites are dispersed in a continuous polymer matrix. The individual one nanometer thick clay layers of an exfoliated nanocomposite are separated by average distances that depend on loading. Exfoliated polymer-clay nanocomposites form when adjacent clay nanolayers are separated by a distance that precludes the possibility of interlayer interaction through the association of gallery cations.
For many technologically important polymers, the monomer intercalation/exfoliation approach is limited because a suitable monomer is not always available. However, various monomers have been used, yielding linear and crosslinked polymer matrices. This technique has been investigated with a number of vinyl monomers including tetrafluoroethylene, cis- and trans- butene, butadiene, 4-vinyl pyrridine, acrylamide, methyl methacrylate, acrylonitrile, and styrene. However, because the adsorption of monomer molecules between clay layers or lamellae is dependent on the dipole moment of the monomer, low yields are generated when any one of the above polymers are physically inserted into clay. The most successful polymerizations within silicate galleries tend to proceed by ring-opening mechanisms, for example the well known epoxy and Nylon-6 nanocomposites.
The use of high performance, high temperature polymers as the organic phase in nanocomposites presents special problems. One of the challenges in the area of high performance polymers is to obtain significant improvements in these classes of polymers, since many already have superb thermal and mechanical properties.
Polyimides are an example of a class of high performance polymers that have been used as matrices for nanocomposites. Since polyimides are used largely for microelectronics it is desirable to reduce the coefficient of thermal expansion, the amount of moisture absorption and the dielectric constant. The preparation of polyimide nanocomposites has shown a significant increase in the gas barrier properties and a decrease in the thermal conductivity. However, the water absorption ratio has not been improved remarkably, and mechanical properties of such nanocomposites have not yet been examined. Additionally, the thermal properties of a polyetherimide, specifically its fire retardant nature, has been examined in both intercalated and exfoliated nanocomposites. Both structures exhibited a delayed decomposition temperature compared to the unfilled polymer, and the immiscible hybrid containing the same amount of silicate showed no improvement, suggesting that formation of the nano-structure is responsible for the increase in the thermal stability.
Preparation of nanocomposites with "traditional" phenolic resins tends to produce an undesirable amount of water during polymerization, have poor processability, require the use of a strong catalyst(s), and have poor mechanical design flexibility.
Thus, there is a need for a compound which can be used to produce intercalated, exfoliated and intercalated/exfoliated nano-structures which possess superior processability, do not require the use of a strong catalyst(s), and have superior mechanical design flexibility. Additionally, there is a need for a monomer that can produce both intercalates and exfoliates in an organo-clay system and which upon polymerization does not produce an undesirable amount of water.